The present invention relates to a channel assignment system for a mobile communication having small zones or cells, in particular, relates to such a system with effective use of frequencies by spacial reuse of densed frequencies, having a channel assignment system with the minumum interference depending upon the interference characteristics depending upon the moving speed of a mobile station.
Mobile communication has a plurality of zones each of which is as small as possible in order to meet with the increase of communication demands. Further, in order to increase the reuse of the frequencies, instead of fixed radio channels, flexible dynamic channel assignment which assigns a channel satisfying a D/U ratio (ratio of desired wave to undesired interference wave) for each communication has been proposed (for instance JP patent application 292450/1988 (JP patent laid open publication 141036/1990)). In that system which uses the densed repetitive frequencies, it must consider the deterioration of communication quality due to interference by movement of a mobile station. The deterioration of communication quality comes from the change of reception level in case of a mobile communication. In other words, the receive levels (field strength) of desired wave and/or undesired wave vary based upon a) the propagation distance (distance depending variation), b) variation by surface structure or profile (median level variation), and/or c) interference variation due to multipath propagation (instantaneous variation).
FIG. 4 shows the explanatory figures of D/U variation and communication quality depending upon moving speed FIGS. 4a, 4b and 4c show the variation of D/U of a high speed moving station (high speed call), a low speed moving station (low speed call), and a very low speed moving station (very low speed call), respectively. The numeral 27 is communication start time, 28 is communication finish time, 29 is threshold level of D/U defining communication quality, 30 is initial D/U measured at the beginning of communication, 31 is D/U variation of a high speed call (median level variation), 32 is median level variation of D/U of a low speed call, 33 is instantaneous level variation of D/U of a low speed call, 34 is median level variation of D/U of very low speed call, 35 is instantaneous level variation of very low speed call, 36 shows the time duration in which the communication quality is less than the threshold level. It should be appreciated in comparing median level variation 31 of a high speed call with median level variation 32 of a low speed call that the quality deterioration occurs in a high speed call because of distance depending variation due to long movement of a mobile station during communication time, but no such deterioration occurs in a low speed call. As for instantaneous level variation, no deterioration occurs when the moving speed is high, but that deterioration cannot be ignored when the moving speed is very low (FIG. 4(c)).
The communication must be disconnected when an interference occurs, unless no control means for overcoming interference operates. Conventionally, when interference occurs, a channel is switched to another channel which is free from interference. However, that conventional system is useless when interference occurs very often, because the load for switching is large when an interference occurs often.